Electrophotographic images are typically produced by first uniformly charging a primary imaging member such as a photoconducting web or drum using known means such as a corona or roller charger. An electrostatic latent image is then formed by image-wise exposing the primary imaging member using known means such as optical exposure, laser scanners, or LED arrays. The electrostatic latent image is then rendered into a visible image by bringing the electrostatic latent image into close proximity to marking particles, alternatively referred to as toner particles, which have been electrically charged so that they will be attracted to the regions of the primary imaging member bearing the electrostatic latent image. Charging the marking particles, which may or may not comprise a colorant such as a dye or a pigment, and bringing the particles into close proximity with the primary imaging member, is generally accomplished using a magnetic brush development station. The marking particles are first rendered suitable for use in a magnetic brush development station by mixing the marking particles with so-called carrier particles. The carrier particles comprise suitable material that will be attracted to the magnets in the magnetic brush development station and may comprise known materials such as ferrites or iron oxides, etc. The carrier particles often comprise various charge agents that impart a controlled charge on the marking particles. The marking particles may also comprise suitable charge control agents so that, upon mixing with the carrier particles, the marking particles obtain an electrical charge of suitable magnitude and sign so as to make them attractive in the proper amounts to the electrostatic latent image in suitable quantities to enable various image densities to be developed in the electrostatic latent image.
In magnetic brush development, toner particles are generally mixed in the sump of the magnetic brush development station with carrier particles to a predetermined level that is measured with a toner concentration monitor. The marking particles are charged by contacting the carrier particles and brought into close proximity to the primary imaging member bearing the electrostatic latent image by rotating the cylindrical shell, the coaxial magnetic core, or both of the magnetic brush development station. The brush is electrically biased in such a manner that, depending on the sign of the charge of the toner particles, the marking particles can be deposited onto the primary imaging member in either the electrically charged or the electrically discharged regions to render the electrostatic latent image visible.
The toned image is next transferred to a receiver, which could be either a final receiver material such as paper, transparency, etc. or to an intermediate transfer member, such as a compliant intermediate transfer member, and then from the intermediate transfer member to the final receiver member. Transfer can be accomplished by applying pressure between the receiver and either the primary imaging member or the intermediate transfer member. More commonly, pressure is applied in conjunction with either an applied electrostatic field or with heat that softens the toner particles. The image is then typically permanently fixed to the final receiver member using pressure, heat, or solvent vapors. In general it is preferred to heat the marking particles to a temperature that exceeds the glass transition temperature of the marking particles so as to render them fluid. Most commonly, the image is fixed to the final receiver by pressing the image-bearing final receiver member against a heated fuser roller. To prevent the final receiver member from adhering to the heated fuser roller, the heated fuser roller is conventionally first coated with a release agent such as a silicone oil. Alternatively, release agents, and in particular wax particles, may be incorporated into toner particles to facilitate release of a fused toner image from the heated fuser roller.
In such systems, it is important that marking particles be electrically insulating when used in conjunction with magnetic brush development and electrostatic transfer. If the particles are not electrically insulating, their charges can change when in contact with the receiver or in the development station. This could impair transfer and development as the applied electrostatic force used to urge the marking particles towards the primary imaging member or to or from a receiver member would vary with the charge on the marking particles. Moreover, even if the charge did not reverse sign or become so significantly altered so as to prevent development or transfer, the control of either or both of these operations could be impeded, resulting in incorrect amounts of marking particles being deposited, with corresponding undesirable density variations and other artifacts occurring.
Conventional electrostatographic toner powders are made up of a binder polymer and other functional additive ingredients, such as pigment and a charge control agent, that are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrophotographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrophotographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
The preparation of toner polymer powders from a preformed binder polymer by the chemically prepared toner process such as the “Evaporative Limited Coalescence” (ELC) offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles having a narrow size distribution (coefficients of variation for particle size (ratio of the standard deviation to the average diameter) are normally in the range of about 15 to 35% or less) are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060; 4,965,131; 6,544,705; 6,682,866; and 6,800,412; and US Patent Publication No. 2004/0161687, incorporated herein by reference for all that they contain. This technique generally includes the following steps: mixing a polymer material and a solvent (and optionally additionally one or more of a colorant, a charge control agent, and a wax) to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent and washing and drying the resultant product.
Performance of electrostatographic toner particles can be impacted by the presence of additives incorporated into the particles, such as colorants, since a portion of the additives tend to reside on the outer surface of the toner, whether prepared by conventional melt pulverization or chemically prepared processes. The triboelectric charging and electrophotographic performance is then influenced by the additives, which may result in distinct toner properties for each toner of a toner set comprising different additives. Typically, incorporated colorants, and other marking particle additives, if electrically conducting or triboactive, such as carbon black, must be totally encased by the polymer that is used to form the marking particles or surface modified in some way. Failure to do so could result in the possibility of an electrically conducting path that would allow the conducting portions of the colorants to contact components of the development station or receiver sheet and thereby alter the charge of the marking particles. Further even if not electrically conducting, additives such as wax particles may have other detrimental effects if present in substantial amounts on the marking particle surface, such as handling and flow properties of the particles. This requirement for effective encasing of such additives can require relatively large quantities of binder polymer for each marking particle relative to the incorporated colorant or other additives, and is undesirable.
Relatively smaller marking particles having reduced mass and relatively high additive loading would be more economically desirable since the amount of toner required to reach aim image density would be reduced. This reduced toner laydown would in turn advantageously reduce required fusing energy and enable high quality printing applications.
The use of porous toner particles in the electrophotographic process can potentially also reduce the toner mass in the image area by allowing for the encapsulation of a high concentration of toner additives in the pores. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Toner particles having an elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost of the print and improve the print quality.
US Publication No. 2005/0026064 describes a porous toner particle. However control of particle size distribution along with the even distribution of pores throughout the particle is a problem. Further, particles with functional additives present primarily in discrete internal pores, and the same being substantially absent from the external particle surface, are not taught.
U.S. Pat. Nos. 3,923,704; 4,339,237; 4,461,849; 4,489,174; and EP 0083188 discuss the preparation of multiple emulsions by mixing a first emulsion in a second aqueous phase to form polymer beads. These processes produce porous polymer particles having a large size distribution with little control over the porosity. This is not suitable for toner particles. Further, particles with additives present primarily in discrete internal pores, and substantially absent from the external particle surface, are not taught.
An object of the present invention is to provide porous polymer particles, and in particular porous toner particles, with additives present primarily in discrete internal pores.
A further object of the present invention is to provide such porous particles where the additive is substantially absent from the external particle surface.
A further object of the present invention is to provide such porous polymer and toner particles with a narrow size distribution.
A still further object of the present invention is to provide a process that produces such porous polymer and toner particles reproducibly and having a narrow size distribution.